Twenty one exclusive mutations in the human equilibrative nucleoside transporter-3 (hENT3), a nucleoside transporter predominantly localized in mitochondria cause a spectrum of human genetic disorders with wide-ranging skin and musculoskeletal disease manifestations (e.g. scleroderma, hypertrichosis, hallus valgus, short stature, etc.). Although there are intriguing similarities between the hENT3-spectrum disorders and mitochondrial disorders, the mechanistic involvement of hENT3 in the initiation, progression, and perhaps in the intervention of these disorders is not understood. Our long-term goal is to identify the molecular pathogeneses of hENT3 spectrum disorders. The overall objective of this R03 grant application is to prove/disprove the concept that abnormalities of mitochondrial physiology are responsible for hENT3 spectrum disorders. Specifically, it is our central hypothesis that interference with mitochondrial nucleoside transport is the molecular explanation for the physiological, biochemical, and clinical manifestations of hENT3-spectrum disorders. This hypothesis is based on preliminary data which show that all hENT3 disease mutations severely impair mitochondrial transport of nucleosides, mitochondrial localization, and/or the stability of hENT3 protein. The rationale underlying the proposed research is that proof in mice that dysfunctional mitochondrial nucleoside transport is the root cause of hENT3-spectrum disorders would provide an experimental model in which to subsequently investigate, in depth, the molecular pathogeneses and potential remedies for these diseases. This central hypothesis will be tested by pursuing two specific aims. Specific aim 1 will determine the role of ENT3 in mitochondrial path physiology. The working hypothesis is that ENT3 will significantly influence in vitro and vivo mouse mito- chondrial functions. The hypothesis is based on our own preliminary data which identify the reduction of mito- chondrial nucleoside transport in hENT3 spectrum syndromes. Cells derived from human disease patients and mENT3 KO mice will be utilized to evaluate mitochondrial path physiology. Specific aim 2 will determine the occurrence and rescue of pathologic changes in mENT3 KO mice. The working hypothesis is that the abnormalities discovered in mENT3 KO mice will closely mimic those seen in hENT3 disorders and that the restoration of mitochondrial transport functions will help in reversal of disease pathology. The hypothesis is based on the similarities between the clinical manifestations of hENT3 and mitochondrial disorders and on the spatial synchrony of mENT3 developmental expression. Comparisons of hENT3 disorder manifestations with the mouse-equivalent of those manifestations will characterize the mouse model. Our studies will validate a crucial disease model and test the concept that interference with mitochondrial nucleoside transport could result in the types of abnormalities seen in hENT3 disorders. This contribution would be significant because the resultant model would enable subsequent mechanistic investigations. The proposed research is innovative because it would enable applicability to multiple skin and musculoskeletal disorders.